This invention relates to injection devices for internal combustion engines. In particular it relates to a unit injector which would be associated with an individual cylinder in a compression ignition type internal combustion engine.
Conventionally, compression ignition engines of a diesel type operate with a fuel pump adapted to communicate fuel to each individual cylinder in response to a timing apparatus so that efficient engine operation occurs. Although fuel pumps for such diesel engines are well advanced, they have certain disadvantages. Because fuel lines are required to conduct fuel from the pump to the cylinders and since this requires a given amount of time, injector timing is delayed as engine speed increases. This requires a variable timing arrangement in the fuel pump drive to advance the timing as engine speed increases. Furthermore, since the high pressure lines contain a relative high volume of fuel, the pump discharge is sensitive to changes in fuel properties and control of injection is less positive than desired.
Accordingly, it is appropriate to pressurize the fuel at the cylinder for injection therein in response to a pumping stroke. Such a procedure overcomes the problems of variations in the pressure, temperature, and other handicaps associated with a central fuel pump arrangement. The unit injector includes a high pressure fuel pump and an injector mechanism in a single housing which communicates with the combustion chamber of an engine through a conventional nozzle. Fuel is provided the unit injector through a low pressure fluid transfer mechanism while timed pumping is provided at the individual injector pump. From the manufacturing standpoint it is advantageous to design a relatively symmetrical unit injector to avoid machining non-symmetric scrolls used in most injectors. Furthermore, such symmetry avoids accurate orientation upon installation.
The discharge rate, or fuel injection quantity, of most presently available unit injectors is usually controlled by a scroll-rack similar to that generally employed by conventional fuel pumps. Use of the rack complicates the housing and requires that a unit injector be precisely located in the cylinder head.